In many printing and processing applications lasers or laser diodes are used to heat or illuminate a target using a scanning and modulated beam. The beam is scanned in two dimensions while the power is modulated at high frequency to achieve the required patterns on the target.
As an example, a vertical-cavity surface-emitting laser (VCSEL) is a type of semiconductor laser diode with laser beam emission perpendicular to the top surface, contrary to conventional edge-emitting semiconductor lasers (or in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer.
Application of VCSEL technology allows a new approach to increase the total speed while even reducing the costs of the laser source. As an example, a line of separately controlled pixels can be built, while scanning is done in one dimension only, similar to the scanning process used in document flatbed scanners. Such pixelated sources have been implemented and next steps are to increase size and power according to desired requirements.
However, when using such concept to full extend, a large number of separately controlled VCSELs is required. At the same time, increasing operation speed also means that total power of each laser diode needs to remain very high. As an example, if a pitch of 100 μm and a current level of 0.5 A at 2V are required for each pixel, this adds up to 1500 A of total current for 30 cm of line length. The sheer number and diameter of such wiring would make the solution impossible to be built. Conventional approaches to solve this problem suggest using multiplexed wiring, e.g., 55 positive and 55 negative connections for the 3000 pixels. For such a solution the on time of one pixel is limited to 1/55 of the time and the current during this time is 55 times the average. It can easily be seen that such an extreme pulsing is not within the capabilities of VCSEL chips and that also here the wiring of 110 wires at a current of 27.5 A is still demanding a lot of diameter.
In display solutions a local transistor adds a separate control to each pixel. In that way only the control status signal needs to go separately (or by serial or multiplexed ways) to each pixel and power is applied globally. Still also this solution can not easily be scaled to the high power density used in VCSEL arrays.